1. Field of the Invention
This invention relates to a process of recovering substantially anhydrous zirconium tetrafluoride from an aqueous solution of hydrofluoride acid and zirconium fluoride.
2. Discussion of Prior Art
U.S. Pat. No. 2,635,037 discloses a process of producing zirconium tetrafluoride wherein zirconium fluoride which contains water of crystallization is produced from ZrOCl.sub.2 .multidot.8H.sub.2 O by its crystallizing precipitation from hydrofluoric acid and is dried and then calcined at 500.degree. to 575.degree. C. to produce anhydrous ZrF.sub.4. That process has various disadvantages. Whenever a starting material consisting of ZrOCl.sub.2 .multidot.8H.sub.2 O is not available, that compound must be produced as an intermediate product. This involves additional costs and ecological problems.
The solution remaining after the separation of the zirconium fluoride contains all chlorine ions which have been introduced as well as unprecipitated zirconium and surplus fluorine which has been fed. The processing of that solution is very difficult and inevitably involves losses of Zr and F. Additionally the recovery of hydrofluoric acid is difficult because a separation from the hydrochloric acid is required initially. Finally, the intended calcining at relatively high temperature involves an unnecessarily high energy consumption.
U.S. Pat. No. 2,639,218 discloses the recovery of anhydrous zirconium tetrafluoride in a three-stage process, wherein ZrO.sub.2 or ZrOCl.sub.2 .multidot.8H.sub.2 O is dissolved in an aqueous solution of hydrofluoric acid and the reaction product is dried and subsequently calcined. In case the starting product used in that process is the same as in the process according to U.S. Pat. No. 2,635,037 the same disadvantages arise. If the starting material consists of ZrO.sub.2 or of metallic zirconium waste, the dissolved fluoride must be recovered by a crystallizing drying step. In that connection a disadvantage resides in that the zirconium fluoride is recovered in the form of very coarse lumps and adheres very firmly to the walls of the drying equipment. The removal from the wall and the disintegration, which is required for an economical calcining, involve unreasonably high costs, particularly because zirconium is a very hard material.
For this reason it is an object to provide a process for the recovery of substantially anhydrous zirconium tetrafluoride from an aqueous solution of hydrofluoric acid and zirconium fluoride which process is free from the disadvantages of the known processes and which can be carried out much more economically, in a simpler manner and with a lower consumption of energy.